1. Field of the Invention
The present invention relates to a charging method and apparatus for transfer in an electrophotograph device.
2 Description of the Related Art
In an electrophotograph device, an electrostatic latent image is formed on a photosensitive body (electrographic recording medium) such as a uniformly charged selenium drum. Toner, of a predetermined color (including black), is charged with charges homopolar to the charged selenium drum, and then applied to the electrostatic latent image to develop it. The developed colored toner is then transferred onto an object (which will be referred to hereinafter as a transferring object), such as plain paper copier (PPC) sheet, e.g., a sheet of paper, by moving the transferring object past a charger operatively positioned relative to the object. This electrophotographic technology is widely used, for example, in image processing apparatus, such as copying machines or transfer type of facsimiles.
In the prior art, the transferring object is continuously charged by applying a constant direct-current high voltage to the charger as the object moves therepast. Accordingly, the charge per unit area of the transferring object fluctuates in accordance with large or rapid changes in speed of the transferring object past the charger. This change in the amount of charge decreases transfer efficiency, and also produces unclear or blurred images.
In a transferring charging process, the transferring object is charged with a polarity different from the charge of the toner applied to the latent image formed on the electrographic recording medium, so that the toner is transferred from the medium onto the transferring object. For example, when the transferring object is fed at relatively high speed, the transferring object may not be charged sufficiently resulting in an incomplete transfer of toner.
On the contrary, when the transferring object is fed at relatively low speed, the transfer charges are injected through the transferring object into the toner thereby changing the polarity of the toner charge to the polarity of the charge applied to the transferring object. This results in separation of the toner applied to the transferring object. In particular, if movement of the transferring object stops, then the possibly of injecting charges into the toner and, consequently, the separation of the toner from the transferring object becomes large.